1. Field of the Invention
This invention relates to pressure relief valves for vessels containing pressurized fluids.
2. Review of the Art
In my U.S. Pat. No. 4,856,547 issued Aug. 15, 1989, I disclose an emergency pressure relief valve for a vessel containing pressurised fluid which comprises a valve body defining a valve passage through the body from a first end communicable with the interior of the vessel to a second end communicable with the ambient atmosphere, a valve seat surrounding the passage, a flap valve, a shaft supporting the flap valve for rotation about an axis perpendicular to a longitudinal axis of the passage between a first position closing the passage and in fluid tight engagement with the valve seat and a second position within the body in which it does not substantially obstruct the passage, and spring means acting between the shaft and the body to urge the flap valve into said first position with a torque sufficient to retain the flap valve in said first position when the pressure difference between the first and second ends of the passage is less than a predetermined value, the spring means including a spring disposed for exposure to the thermal ambience of the vessel to which the valve is applied, the spring being made of a material which loses its spring temper on exposure to a thermal ambience likely to result in substantial weakening of walls of the pressure vessel, whereby substantially to reduce the total closing torque applied by said spring means to the valve after exposure of the spring to such a thermal ambience.
Preferably the spring means comprises multiple springs acting in parallel on said flap including a first spring which is the spring exposed to the thermal ambience of the vessel, and at least one further spring sufficiently protected from the thermal ambience of the vessel to maintain a predetermined minimum closing torque on said flap valve after said first spring has lost its spring temper.
Testing has shown this valve to function satisfactorily, but there can be difficulty in obtaining satisfactory seating of the valve during manufacture, and in maintaining this seating during installation and in service, and over the full range of operating temperature of the valve.